This invention relates generally to a tank which may be used for example on a ship such as an oil tanker, and more particularly concerns a tank which is expandable into the cargo space of such a ship yet which is physically separated therefrom. In recent years the carriage of oil by sea has grown to massive proportions, both in terms of the amount of oil that is carried and in the size of the ships that carry it. In general, the bulk of the oil is transported over long distances, from where it is produced to where it is consumed. Special ships known as oil tankers are used for carrying the oil. After discharge, the oil tankers usually return empty for the next load of oil.
Oil tankers are very safe and responsive ships from stability and manoeuvering points of view when in the loaded condition. However, they can be very unstable and practically unmanoeverable when in the fully unloaded condition. To overcome these defects oil tankers when in the unloaded condition are ballasted (i.e. loaded) with sea water to approximately one third of their maximum cargo capacity, for the return journey to load another oil cargo.
Until recently this ballast sea water was loaded directly into the nearly empty oil cargo tanks after these had been cleaned as far as practicable, and this is usually still the case with smaller oil tankers of less than 70,000 tons deadweight. As oil tanks are hard to clean, the system is prone to serious defects, in particular danger of pollution of the sea if the ballast water is discharged overboard prior to loading another oil cargo. For this reason many shipowners now fit separate ballast tanks used solely for the carriage of sea water ballast when the ship is returning to load. This segregated ballast system is practically 100 percent effective from a pollution prevention point of view, but it is very costly to the shipowner in that he has to provide the extra ballast tank capacity, which cannot be used for the carriage of cargo when the ship is loaded. As many governments now favour the segregated ballast system for ships in their territorial waters, the shipowner is faced with large constructional and operational costs for the segregated ballast tanks.
It is with a view to overcoming the above problem, while still retaining a 100 percent pollution free operation, that the system according to the present invention has been devised.